What Causes Wildfires?

The wildfire season started early this month in the northwestern U.S. and the huge blaze still burns in Fort McMurray Canada after three weeks of fighting the flames. What causes wildfires on this scale, and how can we prevent them? 

Sabrina Stierwalt, PhD
5-minute read
Episode #193

Although three weeks have passed since the fire started, there remain over 1,000 firefighters and 50 helicopters in Fort McMurray, Canada to fight the wildfire raging there. One thousand more firefighters are expected in the next week and the fire remains “out of control.” The fire covers more than 522,000 hectares, but is no longer considered a threat to nearby communities. Also fortunately, gas and electricity have been restored to the surrounding areas, and residents will be allowed to start returning to their homes in the areas deemed safe starting in June.

Wildfires, also known as forest fires, grass fires, or even bushfires in Australia, are uncontrolled fires that usually happen in so-called “wild” areas uninhabited by people. However, they spread quickly, especially in times of drought, and can extend into areas where there are houses and other agricultural resources. They can burn twice as hot as the surface of Venus and reach higher than 50 meters (over 160 feet).

What Causes Wildfires?

Wildfires are most commonly caused by people, but can also be ignited by lightning or even volcanic eruptions. To grow, wildfires need fuel in the form of trees or other vegetation, so the climate must be moist enough to support plant growth but also involve long, dry heat waves. They are most common in wooded areas in the United States and Canada, as well as Australia and the Western Cape in South Africa. The USDA Forest Service provides an up-to-date map of any active fires in the United States.

In southern California in particular, wildfires are often accelerated by the Santa Ana winds, hot, dry air that blows in from the desert. These powerful, fast-moving winds can help wildfires cover 40 miles in a single day.

What Are the Repercussions of Wildfires?

If the conditions are right, wildfires can also produce what is called a fire tornado or a fire devil or sometimes even a firenado. This weather phenomenon appears to be just what it sounds like: a tornado made of fire, although its formation is more similar to that of a dust devil than an actual tornado.

When hot, dry air rises quickly from the ground it forms a column within which the density of the air decreases with height. The less dense air at the top will cool and dissipate, leaving the column, while more, hotter air gets pulled in from the bottom. Thanks to angular momentum, this creates a vertical vortex where the spinning hot air picks up fire debris like embers, ashes, and hot gases. Although at heights of hundreds of feet they can be terrifying looking, firenados usually only last a few minutes. However, they are fast moving, and so can cause wildfires to spread while causing their own significant damage in their short lifetimes.

One positive outcome from wildfires can be making the conditions just right for the growth of fungi. Although we know that mushrooms grow from formations known as sclerotia, when these growths will actually produce the tasty mushrooms we can eat is sometimes a mystery. Some sclerotia exist for years before they bear fruit. Moderate wildfires, however, are known to inspire the growth of so-called “post-fire morels,” which typically pop up a few weeks after a fire and can continue to grow for up to two years afterward.


About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.